1. Technical Field
The present invention relates to a printer and a printing method, and relates more particularly to a printer that has a tractor unit and a printing method that uses the printer.
2. Related Art
Japan Patent No. 3791423, for example, teaches a dot impact printer equipped with a tractor unit that reduces the occurrence of paper jams and similar problems and reliably conveys continuous paper to the printhead. This type of dot impact printer is commonly used for printing reports using fanfold paper, which is a type of continuous paper that is folded together into a stack at perforations between pages.
Because fanfold paper usually hangs down from the printer and is fed sequentially from below up into the printer, a paper feed mechanism with greater gripping power than in a printer that prints on cut-sheet paper is used in a dot impact printer. The dot impact printer taught in Japan Patent No. 3791423 uses a tractor unit with tractor pins that engage sprocket holes formed along both sides of the paper width in the paper feed direction as a paper feed mechanism with strong gripping power. A paper feed mechanism using this tractor unit is driven by a stepper motor because of the ease of control, ease of stopping the motor, and the ease of holding the paper where it is stopped.
Demand for high image quality and quiet operation in printers that are used for printing on fanfold paper has also increased. However, because the paper is conveyed by a stepper motor in the dot impact printer taught in Japan Patent No. 3791423, the stepping angle of the stepper motor limits print quality, and sufficiently high image quality cannot be achieved. The stepper motor alone is also noisy, and possibilities for further reducing operating noise are limited.
It is conceivable to use an inkjet head as the printhead in order to achieve high image quality, use a tractor unit in the paper feed mechanism to assure sufficient gripping power, and drive the tractor unit with a DC motor that can operate continuously and quietly in order to achieve low noise operation and high print quality when printing on fanfold paper or other type of continuous paper.
However, when conveying continuous paper by means of a tractor unit driven by a DC motor, heat produced by the DC motor continuously feeding the continuous paper causes the temperature to rise, and can result in the DC motor coil burning out. Some means of controlling heat output is therefore required, such as setting a delay time to create a pause in the DC motor drive time, or setting an upper limit to the drive (paper feed) speed. Controlling the heat output of the DC motor is particularly important because the tractor unit requires a relatively large amount of power to pull up the continuous paper hanging down from the printer.
When fanfold paper is loaded in the tractor units, the fanfold paper is set in the tractor units so that the tractor pins of the left and right side tractor units are inserted to the sprocket holes disposed on both sides of the paper width, and the gap between the left and right tractor units is adjusted to substantially the same as the paper width. As a result, the tension applied across the width of the paper may vary if the gap between the left and right tractor units is adjusted every time fanfold paper is set in the tractor units, such as when reloading the continuous paper supply when the paper supply runs out. The load on the DC motor also varies because the force required for the tractor units to pull the paper up is affected by variation in tension across the paper width. More specifically, the load on the DC motor changes every time paper is loaded into the tractor units.
As a result, if heat output is controlled by overestimating the variable load of the DC motor in order to reliably avoid heat damage to the DC motor that drives the tractor units, the delay time described above becomes longer than necessary or the maximum drive speed must be set lower than needed. Because the number of pages printed per unit time therefore cannot be increased according to the actual printer conditions, it is necessary to acquire more accurate DC motor load information in order to increase the printing speed.